1. Field of the Invention
The present invention generally relates to a tool for simulating the motions of a satellite in a visual and interactive manner and, more particularly, to a computer system that displays the sky from the point of view of a satellite along with the fields of view of the satellite's star sensors. The invention allows the user to vary interactively the motions of the satellite so that a star-sensor-based attitude maneuver can be planned.
2. Description of the Prior Art
Many satellites use star sensors to determine their attitude, or orientation in space. Stars are identified by comparing their positions in the sensor with known positions obtained from a star catalog. The positions of the identified stars in the fields of view of the star sensors are used to determine the attitude of the satellite. The process of identifying the first few stars in the star sensors is called "stellar acquisition." It is necessary not only after separation from the launch vehicle, but also after a hardware or software error when the attitude of the satellite must be determined using little or no prior attitude information.
There are a number of star identification algorithms, the simplest of which searches the star catalog for the star closest to the unidentified star. This and other algorithms require that an initial attitude estimate be derived from coarse attitude sensors such as magnetometers or sun sensors before beginning the star identification. Due to the coarseness of this initial estimate, which may be accurate to within only a few degrees, the satellite's onboard software could easily confuse one star with another. For this reason a small star catalog is used consisting of bright and widely separated "acquisition stars." Because there are typically no more than thirty acquisition stars, they are easily missed by the star sensors.
The problem of stellar acquisition consists of planning attitude maneuvers to position the star sensors so they will detect acquisition stars. The star sensors must not be pointed too closely to the sun, moon or Earth, because they are easily damaged when pointed too closely (within twenty or thirty degrees) to a bright object. They also should not be pointed near planets, which can be mistaken for stars. To complicate matters, the Earth occludes different portions of the sky as the satellite orbits.
While star identification is usually performed by the onboard software of the satellite, stellar acquisition maneuvers are usually planned by ground-based human analysts. This planning has proven to be a difficult and time-consuming task even for highly skilled analysts. The following approach is typically adopted. A computer program is executed with the estimated attitude of the satellite as an input parameter. The program determines where the star sensors will be pointing and whether or not any acquisition stars will be detected by them. If not, the program is run again with a different attitude "guess" as the input parameter. This trial-and-error process is repeated until a suitable attitude is found.